1. Technical Field
The present invention relates to systems and methods for pulse oximetry measurements at the wrist, particularly, the present invention relates to a pulse oximetry device that can be worn on a wrist.
2. Discussion of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring physiological characteristics of a patient. Such devices provide patients, doctors, and other healthcare personnel with the information they need to secure the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood characteristics, such as the arterial blood oxygen saturation of hemoglobin (SPO2), and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood at the measurement site during each cardiac cycle. Those skilled in the art will appreciate the pulse oximetry techniques used for obtaining the above physiological parameters which may also be termed photoplethysmography or, in short, PPG.
Pulse oximeters typically utilize a non-invasive optical sensor that detects the light response from within a patient's tissue indicative of the amount of light absorbed within the tissue at the illuminated site. One or more of the above physiological characteristics may then be calculated based upon the amount of the absorbed light. More specifically, the light passed through the tissue is typically selected to be of one or more light wavelengths that may be absorbed by the blood in an amount correlative to the amount of the hemoglobin constituent present in the blood. The amount of light absorbed at different light wavelengths may then be used to estimate the arterial blood hemoglobin related parameters using various algorithms. Pulsatile changes in the volume of the arterial blood at the illuminated site during blood pressure wave propagation alter the intensity of the light response detected by the sensor's photodetector.
The quality of the pulse oximetry measurement depends in part on the blood perfusion characteristics of the tissue illuminated by the light and in part on the magnitude of the pulsatile changes in the blood volume within the illuminated tissue. Pulse oximetry techniques typically utilize a tissue site that is well perfused with blood, such as a patient's finger, toe, or earlobe, on which to place the sensor.
For example, FIG. 1 illustrates a sensor 10 adapted to be placed on a finger 12 of a user, such as a patient, according to the prior art. The sensor 10 includes a clip formed of two portions 14 and 16 adapted to clip and constrain the sensor 10 to finger 12 while pulse oximetry measurements are taken. Sensors of a type similar to the sensor 10 are typically coupled to cables 18 that couple the sensor 10 to monitoring systems adapted to receive and process the signals from the sensor 10. Accordingly, such sensor using in continuous monitoring mode typically requires the patient (or user) to be confined to a certain area, in close vicinity of the monitoring system, thereby limiting patient mobility. In addition, pinch pressure applied by clip portions 14 and 16 on the finger 12 of the patient may overtime feel uncomfortable or become overbearing to the patient to the extent the patient may want to remove the sensor 10 and cease otherwise required monitoring. As a result, such sensors are not suitable for the prolonged and continuous pulse oximetry measurements.
Further, as may occur with any physiological signals measuring device, appearance of artifacts and other anomalies in the measured data can alter and/or degrade the quality of collected data to the extent that data may not be useful for providing reliable indication of occurring physiological processes. In that regard, pulse oximetry devices are no exception, as such devices may generally be prone to artifacts arising, for example, from patient motion, which may be random, voluntary or involuntary. Consequently, artifacts arising out of such circumstances can distort and skew obtained data, ultimately adversely affecting the quality of the pulse oximetry measurements. Although the accuracy and reliability of the physiological signals measurements is in large affected by the amount of blood perfusion, as well as by the distribution of the nonpulsatile blood within a tissue site, an increased or excessive amount of motion artifact can become a significant contributing factor to the overall pulse oximetry measurement. Due to aforementioned facts, reflection geometry of the pulse oximetry measurements may not be applicable to various portions of user's body, such as those characterized as having weak blood perfusion, as well being prone to strong motion artifacts. In addition, such body portions may not be suitable for accommodating pulse oximetry devices employing forward transmission geometry in which light emitters and detector are disposed at opposite sides. In such a configuration, portions of the body from pulse oximetry measurements are desired may have tissue layers that are too thick for the light penetrate, thereby impeding the pulse oximetry measurements.
The following patent documents illustrate prior art pulse and/or oximetry devices that are worn on the user's wrist: U.S. patent documents nos. 2010/056934, 2009/247885, 2010/331709, 2002/188210 and U.S. Pat. No. 6,210,340; Japanese patent documents nos. 2009160274, 20052705443, 2009254522, 2010220939 and 2005040261, WIPO patent document no. 2010/111127, Korean patent document no. 20110006990 and British patent document no. 2341233. These devices use either reflection (at 0°) or transmission (at) 180° modes of light detection. WIPO patent document no. 2011/013132 by the present inventor teaches a system and method for measuring one or more light-absorption related blood analyte concentration parameters, using a photoplethysmography (PPG) device configured to effect a PPG measurement by illuminating the patient with at least two distinct wavelengths of light and determining relative absorbance at each of the wavelengths; a dynamic light scattering measurement (DLS) device configured to effect a DLS measurement of the subject to rheological measure a pulse parameter of the subject; and electronic circuitry configured to temporally correlate the results of the PPG and DLS measurements and in accordance with the temporal correlation between the PPG and DLS measurements, assessing value(s) of the one or more light-absorption related blood analyte concentration parameter(s).